You can change the collision settings on the landscape, and then I have been able to do to aim around me from below the floor, but that means that I can no longer aim at the landscape itself at all!
Possibly you could have different collision for different sides of the landscape?
The probe size is the only way I can tell to lengthen or shorten the spring arm?
I had probe size only on 12 when I started, and increasing or decreasing the value doesnt really help. Changing the probe size does clunkily move the camera a bit closer to the tank, but not enough to avoid clipping the camera through the landscape.
Ah! Figured it out. Thanks for the tip on that springarm Dan, it was what was fouling it up. The camera had its own distance set from the spring arm, which is what was making it poorly manipulated.